FS9721_LP1

Rev. 2.0

36/41

R9

90K

R8

10K

VR3

10K

_

+

RT2

1.8K

J2

RT3

100K

R18

10K

RT4

1M

VR4

10K

T6

K type

OP1O

TSTC

SGND

REFO

FS9721_LP1

RT1

100K

J1

Diagram 36

Temperature Measurement (2)

In the diagram, the connection of the thermocouple is more troublesome, but when the thermocouple is not

connected or damaged, it will display the indoor temperature.

1.In these figures, the Bipolar (T6) is used as a Diode. Since the VBE voltage of T6 decreases by 2~2.5mV as

the ambient temperature increases per 1 degree C, this characteristic is used for cold junction compensation.

Something to be addressed here is that:

Since the bandgap voltage is varied upon every single IC chips, and the mo

△

del used for T6 is different, so (

V/

°C) is also different, therefore, the cold junction compensation is different on each IC. So the RT1, RT2, RT3

and RT4 need to be adjusted properly.

2. The resistor VR4 in Figure 35 and Figure 36 is used to achieve the calibration at 0 degree C, while VR3 is

used to achieve the calibration at the high temperature (such as 100 degree C). Since the temperature

coefficient of the externally connected K-type thermocouple is 40uV/°C, we can adjust VR3 so that the

amplification of the OP is set to about 2.5 times.

3. Please note the following 2 aspects when using Figure 35:

1. Because the internal impedance of K-type thermocouple might affect the compensation voltage, we can

change RT2 and RT3 to 1.8K and 100K accordingly to minimize the influence.

2. When using voltage calibration, because of the calibrator’s internal impedance, it causes the

non-accuracy on calibration. Therefore, the best way to do this measurement is to input the voltage directly

across RT1. In other words, the positive voltage is connected to TSTC, and the negative end is connected to

the intersection of RT1 and RT2.